America wakes up to a bright day for women in politics: the remarkable Nancy Pelosi takes over as Speaker of the House of Representatives; Hillary Clinton is riding higher than ever; numbers of women in both houses nudged up, as did women governors. Women's political profile has never been stronger. But above all, the Democrats won the women's vote overwhelmingly: women voted 55% Democrat to 43% Republican. Not since 1988 have the Democrats won so many women's votes.

But over here it's another story. David Cameron owes his lead in the polls entirely to women's votes. Without them he might have a rebellion in the ranks by now. Does this augur a reversion to old voting habits? It is women who have kept Conservatives in power for most of the time since the suffragettes first won the vote. British women are odd: traditionally, in France, Germany and Italy women lean to the left and men lean rightwards; but in Britain the right only ever won on the women's vote. The suffragettes' achievement made the last century the Conservative century; are women about to do it again?

For the first time, women helped Labour win in 1997, but that support has retreated steadily, and at the 2005 election women and men voted identically. By this summer a Guardian/ICM poll had detected women's defection from Labour. The growing anti-Labour gender gap emerged in June, when the Tories were 1% behind among men but scored an 8% lead among women. This week a Times/Populus poll showed Labour and Tories level pegging among men at 34% - but women voters gave Cameron 37% to Labour's 31%.

That's under Tony Blair, the man who was the magician of women's votes. But when men and women are asked how they would vote between Gordon Brown and David Cameron, the gender divide deepens. Men prefer Brown by 3%, but women give Cameron a much heftier lead over Brown than Blair. Wise pollsters are rightly sceptical about these hypothetical "if x was leader" questions. People just don't know enough about x as leader until he/she gets there to give an answer that predicts how they will feel about them in future. But all the same, it suggests that Brown will have to pay rapt attention to what women think and what women want: he can't win without reclaiming them.

It is an extraordinary failure by Labour to have lost support from the voters who have gained most from the most female-friendly government yet. Just consider the great investment in maternity leave, nurseries and childcare: every three- and four-year-old now gets a nursery education. In 1997 there was a childcare place for only one in eight children, but now there is a place for one in three. Sure Start has done more for new mothers than any other programme. It was low-paid women who gained most from the minimum wage, while tax credits help millions of families. More has been spent on education and the NHS, with more new schools, hospitals and clinics, than in any other 10-year period. Crime has fallen; there are more police and a new community support force on the streets. But despite all that, women no longer see this as a government on their side.

What's gone wrong? Women hate war, and they hate it more than men do: that held good in voting patterns in the US elections, as it does in opinion polls across Europe. Deborah Mattinson of Opinion Leader Research runs focus groups with women: "They are more upset about Iraq," she finds, yet she sees the varnish start to peel off Cameron: "Women's bullshit radar is more finely tuned that men's." But her focus groups say the government is "stale" and has "run out of steam". John Reid's announcement yesterday of yet more criminal-justice legislation hardly feels like refreshment: Labour's 59 obsessive criminal-justice bills have often been repealed before they have been enacted. Blair and Reid hammer out security, security, security in a bid to outflank the Tories on the right, trying to brand Cameron "soft on crime". Not only is that daft politics and triangulation gone mad; it also doesn't work. Pollsters do find voters frightened and angry about crime, terror and immigration. But a necessary defensive strategy can't become Labour's defining purpose.

There is a curious paradox here: psychological experiments, now pondered by the Downing Street strategy unit, find that people questioned about their political views are influenced rightwards by dark thoughts: a frightening poster on the wall makes people's attitudes move to the right. So the more Blair goes on about security, war and crime, the more he may drive people into the arms of the Conservatives. This political cross-dressing reached dizzying heights of absurdity with Cameron's daft "Let the sun shine in" speech at the Tory conference. It may have been vacuous and inauthentic, but it did make politics look bright. He talks about women, young people and families all the time, while Labour leaders talk tough. Where is Labour's progressive, optimistic, forward-looking hope for the future these days?

Winning back women has to be the project from now on. How can Labour do that? It could start by copying the Democrats in America in promoting more women up front. It has always been a mystery why Labour has been so bad at telling its own best stories. It now needs a woman right at the top who never lets up. Harriet Harman is the only candidate for the deputy leadership who campaigns loudly and unashamedly on women's issues, always a jump ahead on what needs to be done next. "It would be inexcusable if we were to lose the election because we allowed Cameron to win women's votes," she says. When she was elected in 1982 there were only 10 Labour women MPs, to 13 Tory women. Now Labour has 97 women, while the Tories have achieved only four more in all those 24 years. Why has Labour made its 97 women so invisible, she asks? Cameron will almost certainly choose a woman as his deputy to disguise his party's almost total maleness. It could be fatal if Labour puts up two men.

Life is still hard for mothers, and they know it. When I spent time with a New Deal adviser last week, he despaired at trying to persuade employers to offer jobs to suit mothers' hours: there were none in supermarkets, offices or anywhere. If employers were forced to offer all jobs part time, mothers' prospects could be transformed. And that's the case right up the career ladder; highly skilled mothers find no part-time jobs advertised either. Why not have a fight with the CBI about it, so women get to hear? Also fight it to raise the minimum wage so women can earn enough to keep their families. Why should women's jobs be so undervalued? Make extended schools work, with brilliant activities from 8am to 6pm for free. Make childcare affordable; it isn't for most families. Abolish the "provocation" defence for jealous men who kill their wives.

What Labour needs is a high-profile woman campaigner who never lets go, to make sure the policy reviews push these things high up the agenda. If women voters just don't get the message about what Labour does for women, that's because the wrong messengers at the top fail to convince. Mothers listen to mothers: to win, Labour needs its women up front.